Code compliant behaviour of palm kernel shell reinforced concrete

12 Pages·2015·0.32 MB·English

Vol. 6(4), pp. 59-70, May 2015
DOI 10.5897/JCECT2015.0367
Articles Number : B87FA4153161 Journal of Civil Engineering and Construction
ISSN 2141-2634
Technology
Copyright ©2015
Author(s) retain the copyright of this article
http://www.academicjournals.org/JCECT

Full Length Research Paper

Code compliant behaviour of palm kernel shell
reinforced concrete (RC) beams in shear

A. Acheampong1*, M. Adom-Asamoah2, J. Ayarkwa1 and R. O. Afrifa2

1Department of Building Technology, KNUST, Kumasi, Ghana.
2Department of Civil Engineering, KNUST, Kumasi, Ghana.

Received 11 May, 2015; Accepted 26 May, 2015

This paper evaluates the shear strength of simply supported palm kernel shell (PKS) concrete and
normal weight concrete (NWC) beams subjected to four-point loading. The primary variable of the
investigation was evaluation of shear capacity of reinforced PKS concrete and NWC beams with and
without shear reinforcement. Four pairs of reinforced concrete (RC) beams with similar geometrical
properties were designed and tested with two replicate beams per design. Measured shear strengths at
failure were compared with theoretical predictions calculated using BS 8110, ACI 318-05 and EC2. All
but one specimen failed as a result of diagonal-tension. PKS beams exhibited lower ultimate shear
capacities, smaller crack widths but higher displacements at failure. The performance of the EC 2, BS
8110 and ACI 318-08 code equations in predicting the shear resistance of PKSC and NWC beams is also
presented. Based on the test results, it is concluded that shear capacity provisions of ACI give a
conservative estimate of shear capacity of PKS lightweight concrete beams while the EC 2 gives a very
close prediction of the shear capacity of PKS RC beams. The results of this study increases knowledge
on the structural properties of PKS concrete, the efficient use of PKS and reduce the indiscriminate
disposal of the PKS as a waste material.

Key words: Palm kernel shells, normal weight concrete (NWC), Shear strength, BS 8110, ACI 318, EC 2.

INTRODUCTION

The construction industry depends largely on Lightweight aggregate concrete (LWAC) has successfully
conventional materials which are cement, granite and been investigated and used for structural purposes for
sand for concrete production. The increasing demand for many years. This is due to the added benefits to the
concrete in the construction industry using normal weight construction industry in its application. Its use results in
aggregates such as gravel and granite has led to the higher strength to weight ratio, higher tensile strain
depletion of the natural granite deposits at an alarming capacity, superior heat and sound insulation
rate (Emiero and Oyedepo, 2012; Alengaram et al., characteristic, and lower dead loads with smaller sections
2008), while efforts to maintain ecological balance has for structural members (Nahhas, 2013; Yasar, 2003).
also been a major challenge. However, efforts to This and other associated benefits make the use of
substitute granite as coarse aggregate has been a major lightweight concrete (LWC) one of the important
challenge (Adom-Asamoah, and Afrifa, 2010). strategies in reducing the cost of construction in

*Corresponding author. E-mail: [email protected].
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution
License 4.0 International License
60 J. Civ. Eng. Constr. Technol.

developing and underdeveloped countries (Shafigh et al., for the structure to fail in a brittle manner without any
2010; Ndoke, 2006). Lightweight aggregates may be warning if the shear stress exceeds the shear carrying
either artificial or natural aggregates that are produced capacity (Jasim, 2009). Structural design codes have
from a wide variety of raw materials and production therefore made provisions for the determination of safe
procedures. Depending on the production techniques and shear capacities of concrete and have become more
the source of origin, lightweight aggregates vary within stringent on prevention of sudden failure (Mansour et al.,
broad limits (Chi et al., 2003). The use of different 2004; Yang et al., 2003). Jumaat et al. (2009)
materials, ranging from industrial wastes to agricultural investigated the shear strength of four reinforced oil palm
waste, and other naturally occurring lightweight shell foamed concrete (OPSFC) beams with a target
aggregates for concrete production have been reported in density of 1600 kg/m3 and a 28-day compressive strength
literature. Palm kernel shells (PKS) are agricultural solid of about 20N/mm2. It was concluded that OPSFC with
waste obtained from cracking the fruits of the palm tree. shear reinforcement failed in flexure mode while beams
PKS are hard, stony, light and naturally sized, flaky and without shear reinforcement failed in shear modes.
of irregular shapes depending on the cracking pattern of Additionally, it was reported that OPSFC beams had 50%
the nut. Large quantities of PKS are produced by the higher deflections compared to corresponding NWC
palm oil mill but only a fraction is used as fuel for steam beams at ultimate stage. The spacing of cracks in
boilers at palm oil mills. The greater proportion of this by- OPSFC concrete beams were all reported to be closer
product is therefore disposed off in stockpiles in open than those found in NWC beams. Setiawan and Saptono
fields which results in negative impact on the (2012) reported on the shear capacity of reinforced
environment. Exploiting the use of PKS in construction concrete beam with different cross section types of lateral
not only maximise the use of oil palm, but also helps to reinforcement on minimum ratio. It was concluded that
preserve natural resources and maintain ecological the various types of shear reinforcement cross sections
balance. Studies (Okpala, 1990; Mannan and Ganapathy, had no significant differences for shear capacity of the
2002) have shown that the use of PKS as aggregates concrete.
can produce structural concrete of compressive strength To fully understand the behaviour of PKS concrete in
of about 20 N/mm2 at 28 days with a density in the range shear, it is of importance that the effect of shear
of 1800-1900 kg/m3. The structural behaviour in relation reinforcement on the behaviour of PKS RC beams in
to flexure and bonding has been reported in addition to shear be investigated. Knowledge of the post-peak
the mechanical properties of PKS concrete (Teo et al., deformation characteristics of shear-critical reinforced
2006; Alengaram et al., 2010). Teo et al. (2006) reported concrete members of PKS lightweight concrete is
that ultimate moments predicted using BS 8110 provides important to better understand the contribution of the
a conservative estimate for PKS concrete beams up to a shear (web) reinforcement and the failure mechanisms
reinforcement ratio of 3.14%. Deflections and crack for structural applications. This study is therefore aimed
widths at service loads were all reported to be within at studying the effect of the amount of shear
maximum allowable values stipulated by BS110. Teo et reinforcement on the shear strength of PKS and NWC
al. (2007) investigated the structural bond and durability RC beams. To this end, the code compliant behaviour of
properties of PKS aggregate concrete. The authors PKS RC beams in shear was investigated using BS
concluded that the experimental bond strength of PKS 8110, ACI 318 and EC 2 to determine their suitability as
concrete was much higher than theoretical bond strength shear models for LWC.
as stipulated in BS 8110. Alengaram et al. (2010)
concluded that the bond between PKS and cement matrix
in tension was weaker compared to the bond between Prediction of shear capacity of RC beams
crushed granite aggregates and cement matrix.
Shear behavior and design of reinforced lightweight The behaviour of RC elements in flexure has been well
aggregate concrete (LWAC) beams in shear is one of the understood such that their flexural strengths can be
most topical issues in the structural behavior of reinforced predicted with reasonable accuracy. Contrary to the
concrete due to its brittle nature (Russo et al., 2004). It is flexural behaviour of structural members, sophisticated
generally assumed that lightweight concrete members approaches have been proposed based on fracture
are weak in shear due to their lower tensile strength. The mechanics, physical models of structural failure, and
shear failure of reinforced concrete beams without web finite element analyses (Song et al., 2010; Oreta, 2004)
reinforcement is a distinctive case of failure which to predict the behaviour of beams in shear. That
depends on various parameters such as shear span to notwithstanding, no single theory is available for
effective depth ratio (a/d), tension steel ratio (ρ), type of estimating the precise shear strength of reinforced
aggregate, strength of concrete, type of loading, and concrete elements (Bentz et al., 2006). This problem has
support conditions (Sudheer et al., 2011). The amount of been attributed to the complex nature associated with the
shear reinforcement has a direct relation on the behavior shear transfer mechanisms especially after cracks are
of structural RC members. This is because it is possible initiated (Song et al., 2010). As a result, the design and
Acheampong et al. 61

Table 1. Different approaches to Shear design.

Approach Shear Strength, Vc Shear Strength, Vs
BS 8110 - 97 0.79 100(cid:12)(cid:13) 1(cid:19)3 400 14 (cid:22)(cid:2)(cid:23) 1(cid:19)3 (cid:12)(cid:13)(cid:15)
(cid:1)(cid:2) = (cid:8) (cid:9)(cid:10) (cid:14)(cid:15)(cid:16) (cid:17) ×(cid:10) (cid:16) (cid:17) ×(cid:10)25(cid:17) (cid:26)(cid:14)(cid:16) (cid:1)(cid:27)= (cid:27)(cid:15) ×0.95(cid:22)(cid:28)(cid:16)

EC 2 - 2004 (cid:1)(cid:29)(cid:30), =!0.12"1+$200(cid:19)(cid:16)%&100'1(cid:22)(cid:2)()1(cid:19)3+0.1(cid:12)*(cid:2)+(cid:30),(cid:14)(cid:16) (cid:1)(cid:29)(cid:30),(cid:27) =(cid:12)(cid:27)(cid:13)-.(cid:22)(cid:28)-(cid:16) /01

ACI 318 - 08 ′
(cid:1)(cid:2) =!0.163$(cid:22)(cid:2)+17'-(cid:1)(cid:23)(cid:16),(cid:14)-(cid:16) (cid:1)(cid:27) =(cid:12)(cid:1)(cid:22)(cid:28)0(cid:16)
4(cid:23) (cid:27)
V: Shear strength provided by concrete; , f , f : Concrete compressive strength; b : Web width; d: Effective depth; V: Shear force;
c ck cu w u
M: External moment; , : Longitudinal reinforcement ratio; N : Axial force; A: Cross sectional area of concrete; : modification
u ED c
factor reflecting the reduced mechanical properties of lightweight concrete; Z: Lever arm; f : tensile strength of longitudinal
yt
reinforcement; V: shear strength contributed by shear reinforcement.
s

behavior of structural concrete to shear is an important configurations and the material properties (Juan, 2011).
and ongoing area of research in structural concrete. Considering the complex nature of shear resistance,
However, research has revealed that the resistance of design codes consider the shear capacity of concrete
reinforced concrete members to shear is the summation members from two mechanisms: design shear capacity of
of several internal shear transfer mechanisms (NCHRP, the concrete without shear reinforcement, V and the
c
2005). Some (Hassan et al., 2008; Taylor, 1974) suggest contribution of shear reinforcement, V as shown in Table
s
that transfer mechanisms include shear in the uncracked 1.
compression zone of the beam (ranging between 20% For lightweight aggregate concrete (LWAC) members,
and 40%), aggregate interlock mechanism or interface (BS 8110-2, 1985) adopts the same design parameters
shear transfer (ranging between 35% and 50%), dowel as that of normal weight concrete members for concrete
action of the longitudinal reinforcement (ranging between grades greater than 25 MPa. Meanwhile the concrete’s
15% to 25%), whilst others Jung and Kim (2008) attribute design stress (V ) is taken as 0.8 times the values for
c
the transfer to other factors such as residual tensile normal weight concrete if the grade of concrete is greater
stresses across the crack, and the presence of shear than 25 MPa. This factor is also imposed on the
reinforcement. It is therefore believed that a major maximum limit of shear stress that a section can be
component of the shear transfer in the fractured interface subjected to. That is 0.63f or 4 MPa whichever is lower.
cu
is generated from the frictional forces that develop across The design for shear using the (ACI 318, 2008) is
the diagonal shear cracks due to "aggregate interlock" based on the computation of the shear strength, V , of
c
which provides resistance against slip (Hassan et al., the concrete beam cross-section and the maximum
2008). Generally, tensile stresses due to external loading shear, V , that the beam will resist. A reduction factor of
u
will exceed the tensile capacity of the concrete, leading to 0.85 is adopted for sand-lightweight aggregate concrete
flexure cracks. Once these cracks form, the beam no while a factor of 0.75 is assumed for all-lightweight
longer responds to loads in an elastics manner. Shear is aggregate concrete.
then transferred through the intact concrete in the Contrary to the design provisions of ACI 318, the
compression zone above the flexure cracks. If the shear provisions of the EC2 consider a semi-empirical equation
span-to-depth ratio is small, arching action and a for members which do not require shear reinforcement.
compression strut may also develop to carry part of the This equation considers size effect of beams instead of
shear. the shear span to depth ratios. Values of C for
rdc
Most design codes however use empirical equations for lightweight concrete is also reduced from , for
simplicity. Some design codes like the ACI limits the
normal weight concrete to for lightweight
shear capacity of a member to the onset of diagonal
cracking of the beam, regardless of the ultimate shear concrete. Another reduction factor which depends on the
load which may usually be higher than the cracking load. density of the lightweight aggregate concrete used is also
This is because the extent to which a member continues imposed on lightweight concretes. The coefficient is
to resist shear after the onset of diagonal cracking is taken as 60% of the normal weight concrete by the ratio
unpredictable and depends largely on the loading of the upper limit of the appropriate density class to the
62 J. Civ. Eng. Constr. Technol.

Table 2. Physical properties of aggregates.

Properties PKS (LWA) Granite (NWA) Limits
Maximum aggregate size, mm 14 14 0
Shell thickness, mm 1 – 5.9 - 0
Specific gravity, saturated surface dry 1.35 2.65 < 2.4 - 2.8
Aggregate impact value (AIV), % 3.01 13.5 ≤ 25
Aggregate crushing value (ACV), % 5.30 25.7 ≤ 30
Los Angeles Abrasion Value (AAV), % 4.73 19.6 ≤ 30 and ≤ 50
24-hour water absorption, % 18 0.68 25%
Flakiness Index (%) 63.2 31 40 – 50
Elongation Index (%) 16.6 22 40 – 50

Table 3. Mix proportions of concrete.

Mix ratio
Mix designation
Cement content, Kg/m3 w/c Cement/sand/aggregates Superplasticizer (%)
PKSC, P 550 0.40 1:1.3:0.70 1
NWC, N 440 0.50 1:1.7:2.5 0

Table 4. Properties of beam specimens and concrete strengths.

Beam size Effective Age at Compressive Flexural Spacing of
Beam designation B x D depth d testing strength, strength, fr stirrups
(mm) (mm) (days) f (N/mm2) (N/mm2) (mm)
c
P0 110 × 225 193 28 23.6 3.60 -
P200 110 × 225 193 28 22.6 3.70 200
P250 110 × 225 193 28 23.4 3.71 250
P300 110 × 225 193 28 23.1 3.45 300
N0 110 × 225 193 28 28.7 4.1 -
N200 110 × 225 193 28 29.5 4.1 200
N250 110 × 225 193 28 30.3 4.3 250
N300 110 × 225 193 28 29.8 4.3 300

density of normal weight aggregate concrete (2200 the PKS concrete was obtained from trial mixes due to the
kg/m3). The reduction factor becomes . comparatively different nature of the PKS aggregates (Table 2), to
obtain a target design strength of 25 N/mm2. Meanwhile, the granite
concrete was designed for a target strength of 25 N/mm2 using the
UK’s Department of Environment (1998) mix design method. Sika
EXPERIMENTAL PROCEDURE viscocrete, high-range water reducing admixture (superplasticizer)
was used to improve the workability of the PKSC mix since the
Materials and beam identification water/cement (w/c) was kept low to obtain the target strength. The
mix proportions of both PKSC and NWC are presented in Table 3.
Ordinary Portland cement with a 28-day compressive strength of Eight reinforced concrete beams (four PKS and four granite
42.5N was used in the study. The fine aggregate used in the study reinforced concrete beams) were cast and tested in the Civil
was river sand. The coarse aggregates were crushed palm kernel Engineering Concrete Laboratory of KNUST. Four of these beams
shells (PKS) with a maximum size of 12.5 mm (for PKS concrete) were made of PKS concrete (P0, P200, P250 and P300) and the
and granite (for the normal weight concrete). The shells were other four, normal granite concrete (N0, N200, N250 and N300). All
flushed with water to remove dust and other impurities. The the beams were of the same length of 2000 mm with the same
aggregates were oven dried and the physical properties were cross-sectional dimension of 110mm (width) and 225 mm (depth).
determined (Table 2). Due to the high water absorption properties Each beam is designated by a letter and a number. The letters (P
of the PKS aggregates (Table 2), they were pre-soaked for 24 or N) denotes the type of concrete whilst the number (0, 200, 250 or
hours and subsequently air dried prior to its use. Mix proportions of 300) denotes the spacing of shear stirrups (Table 4). Each concrete
Acheampong et al. 63

P
2R6 -All beams
P/2 P/2
110
3R16 225mm
600mm 600mm 600mm
1800mm
P/2 P/2 3R16

Figure 1. Beams without shear reinforcement, P0 and N0.

P
R6@200, 250 or 300 c/c P/2 P/2 2R6 2R6 -All beams
110
225mm
3R16
600mm 600mm 600mm
P/2 P/2 3R16

Figure 2. Beams with shear reinforcement (P200, P250, P300 & N200, N250, N300).

Plate Loading head of testing machine
Loading beam
Roller Steel plate
Test specimen
Roller
Steel support
Dial gauge
frame
100 600mm 600mm 600mmm 100

Figure 3. Schematic sketch of experimental set-up.

type had one beam (P0 and N0) without web reinforcement. The Testing of beams
other three beams of each type of concrete (P200, P250, P300,
N200, N250 and N300) had web reinforcement in the spacing of The beams were simply supported on a stiff steel frame in the Civil
200, 250 and 300 mm, respectively. Both categories of beams were Engineering Laboratory of the KNUST, Kumasi. A hydraulic
provided with nominal hanger bars of 2R6. Deformed mild steel actuator under crosshead displacement control was used to apply
bars of mean yield strength 271.2 N/mm2 were used for the loads through a steel spreader beam. A four point loading
longitudinal reinforcement and shear reinforcement (stirrups). The configuration (Figure 3) was used for the test. The spreader beam
geometrical properties and reinforcement details of the beams are had sufficient bending capacity and stiffness to avoid excessive
shown in Figures 1 and 2. Companion concrete specimens of 150 × deformation and yielding before failure of the test specimens.
150 × 150 mm and 100 × 100 × 500 mm were cast to study the Beam deflections at mid-span for a steady loading rate of 0.2
compressive and flexural strengths respectively. Curing was done kN/s were measured with the aid of a dial gauge with a 0.001 mm
using hessian mat spread on the beams in the open temperature accuracy fixed at the bottom of each beam. Cracks were marked on
with regular watering until 28-days. the sides of the specimens as they developed, in order to assess
64 J. Civ. Eng. Constr. Technol.

Table 5. Test results of cracking loads and service load crack widths.

Cracking loads, kN
Service Crack width Crack width No. of Ultimate
Beam no. First flexural First shear shUelatirm foartce e, Postc draiacgko nal Loads, Vsl at service at failure, cracks at deflection, δc Fmaoilduerse
crack, V crack, V (kN) loads, (mm) (mm) failure (mm)
fc cr V resistance
u
P0 10 28 50 22 33.33 0.22 0.29 13 6.86 FS/DT
P200 12 42 72 30 48.00 0.24 0.32 24 12.46 FS/CC
P250 10 38 68 30 45.33 0.215 0.34 23 11.60 FS/DT
P300 8 30 62 32 41.33 0.235 0.36 21 9.55 FS/DT
N0 14 40 60 20 40.00 0.245 0.31 22 6.22 FS/DT
N200 16 50 74 24 49.33 0.255 0.33 19 13.48 FS/DT
N250 14 48 72 24 48.00 0.24 0.45 18 8.05 FS/DT
N300 12 42 66 24 44.00 0.32 0.72 16 7.95 FS/DT

FS: Flexural shear; DT: Diagonal tension; CC: Concrete crushing.

the first flexural and shear cracks, and crack widths at properties of the PKS concrete and normal weight initiated at 10 kN which represent 20% of ultimate
tension steel levels. Observation of cracking was concrete beams tested at the age of 28 days are load for the palm kernel shell concrete beams
performed visually while the crack propagation and crack
presented in Table 4. The results show that the without shear reinforcement (P0) compared to 14
pattern were marked by hand. Selected crack widths were
compressive strength of PKS concretes are higher kN (23% of ultimate load) for the normal weight
measured using a crack microscope of optical
magnification X10 and reading to 0.02 mm. Initiation and than the minimum required strength of 17 N/mm2 concrete beams without shear reinforcement (N0).
propagation of both flexural crack and shear cracks were for structural lightweight concrete (ASTM C330, First flexural cracks varied from 12.9 to 16.7% of
closely observed and recorded against corresponding 1999). Considering the physical properties, the the ultimate for PKS concrete beams with shear
applied loads. lower compressive strength could imply that the reinforcement (P200, P250 and P300). First

compressive strength of the PKS ultimately flexural cracks for the NWC beams with shear
depends on the bond between the aggregates reinforcement (N200, N250 and N300) varied from
and the cement matrix. Thus, improving the bond 18.2 to 21.6% of the ultimate load. It is reported
RESULTS AND DISCUSSION
between the PKS aggregates will result in that the type of aggregate in concrete can

increased compressive strength of the PKS influence the characteristics of the reinforced
Properties of aggregates
concrete. concrete members in shear (Adom-Asamoah et

al., 2012). This was evident as failure loads of
The results of the physical properties of both PKS NWC beams were comparatively greater than
and crushed granite aggregates are presented in General behaviour of beams those of PKS concrete beams (Table 4). In all
Table 2. The physical properties of the aggregates beams, it was seen that increasing the amount of
conformed to the minimum requirements of (BS Table 5 shows the values for cracking loads, shear reinforcement (stirrups) led to an
882, 1992), however, the PKS aggregates appear ultimate loads and the maximum deflections. In all enhancement of cracking strength of the beams
stronger than the granite aggregates used. The beam specimens, flexural tension cracks as observed by others researched on other non-
results of the densities and mechanical properties propagated first in the pure bending zone followed conventional aggregates for concrete (Adom-
such as compressive and flexural strength by shear cracks in the shear zone. Flexural cracks Asamoah and Afrifa, 2013).
Acheampong et al. 65

Figure 4. Load-deflection curves for PKSC and NWC beams.

Load-deflection, cracking and failure behaviour beams. This could be attributed to the lower modulus of
rupture of the PKS beams (Table 4). Diagonal shear
First flexural cracks were observed at lower loads among loads varied from 48.4 to 58.3% of ultimate loads for PKS
the PKS concrete beams when compared to the NWC concrete beams and 63.6 to 67.6% of ultimate loads for
beams. This indicates a lower bending strength of PKS NWC for beams with shear reinforcement. The post-
concrete beams as shown by the results of the modulus diagonal cracking shear resistance of PKS concrete
of rupture test (Table 4). In all instances, the ultimate beams without shear reinforcement were comparatively
deflections for PKS concrete beams were higher than higher than that of corresponding NWC beams (Table 5).
corresponding NWC concrete beams (Table 5). This may Diagonal shear cracks were observed at about 56% of
be attributed to the good energy absorbing quality of the the ultimate load for the PKS concrete as compared to
PKS aggregates derived from the low AIV and ACV 66.7% of ultimate loads for corresponding NWC beams.
typical of PKS compared to NWC (Teo et al., 2007). The At the post-diagonal cracking, beams without shear
initial straight portion of the curve, before first crack, reinforcement derive their shear resistance from
indicated that the pre-cracking stiffness of the beams aggregate interlock mechanisms, and the dowel action
were always higher than those of PKS independent of the between the aggregates and the longitudinal
type of beam and amount of shear reinforcement. At this reinforcement. The results show good aggregate interlock
stage, the beams behaved elastically until the onset of in the PKS beams compared to the NWC beams, thereby
cracks, after which shear is transmitted through the contributing to the higher post-diagonal cracking shear
concrete section above the flexural cracks. The onset of resistance. More so, it could be deduced from Table 2,
cracking in the concrete is thus controlled by the bending the palm kernel shell concrete were able to absorb more
strength of the material itself and the compressive loads even after cracking as compared to the normal
strength of the concrete. The beams without stirrups (P0 weight concrete probably due to the lower ACV of 5.3%.
and N0) failed at a much lower load and had significantly In addition, after the post-diagonal cracking, the good
less deflection than the beams with shear reinforcement bonding between concrete and the longitudinal
(Table 5). Shear transfer through the shear reinforcement reinforcement resulted in the transfer of stresses to the
was activated once the concrete section was no longer steel; leading to the utilization of the tensile strength of
able to resist the applied stresses. Consequently, the the steel to achieve equilibrium in the composite section
ultimate deflections were proportional to the amount of (Nejadi and Gilbert, 2004).
shear reinforcement (Figure 4). The number of cracks in PKS concrete beam without
The cracking loads and corresponding service loads shear reinforcement (P0) was 13 compared with 22
and crack widths are summarized in Table 5. Generally, cracks in NWC (N0). The average number of cracks in
all beams failed in shear mode except P200 which failed the PKS beams with shear reinforcement (P200, P250
as a result of flexural shear. Cracking of PKS beams and P300) were greater than those of corresponding
were along the convex surfaces of the shells which NWC beams. The amount of shear reinforcement was
shows a weak bond between the PKS aggregates and directly proportional to the number of cracks in both types
the cement matrix. The PKS concrete beams exhibited of concrete specimens at failure. As observed by (Lim et
early flexural cracks compared to the companion NWC al., 2006; Hassan et al., 2008) a higher number of cracks
66 J. Civ. Eng. Constr. Technol.

Figure 5. Crack patterns and flexural shear failure of Specimen P200.

resulted in smaller crack widths for both types of concrete (Sagaseta and Vollum, 2010). Diagonal cracks were far
beams. It is also seen from Table 5 that PKS concrete from the mid-span of the beam, and splitting the bond of
beams developed smaller crack widths compared to that the longitudinal reinforcement all the way to the point of
of corresponding NWC beams. The wider cracks in NWC loading. It was observed that initial diagonal tension
can be attributed to physical characteristics of the normal cracking occurred at a low load in the PKS beams which
weight aggregate as aggregate interlock plays a is related to the flexural strength of the concrete (Juan,
significant role in shear transfer across a diagonal crack 2011). That notwithstanding, PKS concrete beams were
(Adom-Asamoah and Afrifa, 2013). It was expected that able to develop sufficient shear capacity from other
the higher deflections in the PKS beams coupled with the mechanisms to continue resisting increasing loads. In
smooth convex surfaces would result in wider crack addition to the diagonal shear failure, the NWC showed
openings along the convex surfaces. However, the bond failure at the tension side of the beams. This could
required friction developed by the smooth crack surface be attributed to the high stress concentration near the
of the PKS aggregate concrete for shear transfer, is lost support. It is reported that for shear span/effective depth
at a lower crack width compared to normal weight (a /d) ratio greater than 1 but less than 2.5, the diagonal
s
concrete. Since the developed friction is low, the crack crack often forms independently and not as a result of
slips before yielding of the shear reinforcement (Duthinh, flexural crack development (Sagaseta and Vollum, 2010;
1997), thereby leading to a brittle failure. This could have Kong and Evans, 1994).
contributed to the higher post-diagonal cracking Beam specimen P200 failed in a ductile manner and as
resistance but lower ultimate failure loads. Cracks result of concrete crushing/flexural shear (Figure 5).
occurring within concrete do not only reduce the stiffness Vertical flexural cracks propagated from the tension side
of structural members (Duthinh, 1997) but also make it of the beam to the compression zone. Crushing of the
weak with age due to exposure of structural steel to the concrete occurred with associated disintegration of the
environment (Parghi et al., 2008). Therefore, crack widths concrete cover in the compression zone. Once the
have to be controlled to acceptable limits in reinforced concrete crushed, the beam continued to resist shear
concrete members for a variety of reasons, such as, stresses through the shear reinforcement and the dowel
control of deflection, corrosion protection, impermeability, action of the longitudinal reinforcement until the
maintenance of integrity and appearance of the structure. longitudinal reinforcement yielded. Specimen P200
contained the greatest amount of shear reinforcement for
the PKS beams in this study. Given the low compressive
Failure modes strength (Table 3), P200 failed as a result of flexural
shear. This implies that the increased shear capacity
Table 5 presents various modes of failure for all (resulting from high shear reinforcement ratio) of the PKS
specimens. The mode of failure of the test specimens beams led to the flexural shear before reaching its full
was brittle in nature with virtually no warning. With the shear capacity. Jumaat et al. (2009) observed that oil
exception of P200 which failed as result of concrete palm shell foamed concrete (OPSFC) beams with 150
crushing, all beam specimens failed as a result of mm centre to centre web reinforcement failed in flexure
diagonal tension/flexural-shear. Ahmed et al. (1995) compared to OPSFC beams without shear reinforcement
noted that the diagonal tension (shear) failure of concrete which failed in shear modes. Thus increasing shear
is brittle in nature and fails with little or no warning reinforcement in reinforced PKS concrete beams results
especially the failure of beams reinforced only with in flexural failure mode instead of shear mode.
flexural (tensile) reinforcement. Typically, the concrete
rupture completely with wide diagonal cracks which are
widest in the flexural tensile zone of the beam. Ultimate Shear resistance characteristics of PKS and NWC
failure of all beams occurred with the material rupturing beams
along a fresh diagonal crack parallel and independent
from earlier formed cracks. The beams remained To analyze and compare the shear strength of beams,
relatively stable after the formation of the diagonal crack, the ultimate shear load (V ) is normalized to account for
u
which run between the inner edges of the supports the difference in compressive strength between SCC and
Acheampong et al. 67

Table 6. Experimental results and code predictions.

Code predictions – V Shear force ratios, V /V
code code exp
Beam ID Exp. Shear force (kN), V
exp
BS 8110 EC 2 ACI 318 BS 8110 (%) EC 2 (%) ACI 318 (%)
P0 50 36.48 38.95 36.04 73 78 72
P200 72 62.40 63.51 48.05 87 88 67
P250 68 57.22 58.60 45.65 84 86 67
P300 62 53.76 55.32 44.04 87 89 71
N0 60 45.60 49.05 41.90 76 82 70
N200 74 71.52 73.60 53.90 97 99 73
N250 72 66.34 68.69 51.50 92 95 72
N300 66 53.76 63.28 49.90 81 96 76

NC. Since the shear strength is proportional to the spacing of shear reinforcement (Figure 6). for NWC beams with shear reinforcement. It is
square root of the compressive strength of seen that, both ACI 318 and BS 8110 are
concrete (f ) the normalized shear load (V ) was conservative, especially for the palm kernel shell
c n
determined as follows: Comparison of test results and some existing concrete beams. However, the ACI is found to be
code shear models more conservative than the BS 8110. The ratio of
predicted EC 2 values to the experimental values
(1)
Table 6 compares the experimental results and ranges from 86 to 89% for the PKSC concrete
theoretical loads calculated using equations in beams and ranges from 95 to 99% for the NWC
The normalized shear stress (V ) is then Table 1. The performance of BS 8110, EC 2 and beams. EC 2 is found to closely predict the
ns
calculated as: ACI 318-08 in predicting the ultimate shear load of ultimate shear capacity of the PKSC and NWC
both PKSC and NWC beams with and without beams compared to that of the ACI and the BS
shear reinforcement have also been presented. It 8110. For NWC beams without shear reinforce-
(2)
is seen that ACI 318-08 under predicted the ment, the predictions are 70%, 76% and 82% of
ultimate shear capacity of both PKSC and NWC the shear capacity of the NWC beams for the ACI,
Normalized shear load and stress for all beams irrespective of the amount of shear BS 8110 and EC 2 respectively. The predictions
experimental PKSC/NWC beams are tabulated in reinforcement. The ratio of V(ACI 318)/Vexp ranges of PKS beams without shear reinforcement are
Table 7. between 67 and 76% for PKSC and NWC beams 72, 73 and 80% of the shear capacity of the NWC
In general, PKSC beams with shear with shear reinforcement. beams for the ACI, BS 8110 and EC 2,
reinforcement exhibited higher Vns compared to The study shows that the conservativeness of respectively.
NWC beams with shear reinforcement. The the ACI increases with decreasing amount of The shear stress ratios of experimental values
normalized shear stress, Vns also decreased with shear reinforcement, and can be used to safely to to BS 8110 predicted values range from 1.15 to
increasing spacing of shear reinforcement for both predict the ultimate shear resistance of both 1.19 for PKSC beams with shear reinforcement
NWC and PKSC beams. The normalized shear PKSC and NWC beams. The ratio of BS 8110 while the ratio range from 1.04 to 1.23 for NWC
stress of NWC beams without shear prediction to experimental values ranges between beams with shear reinforcement. The shear stress
reinforcement was however, found to be higher 84 to 87% for PKSC beams with shear ratios of the experimental values to EC2 predicted
than corresponding PKSC beams with varying reinforcement and ranges between 81 and 97% values range from 0.98 to 1.11 for PKSC beams
68 J. Civ. Eng. Constr. Technol.

Table 7. Shear stress prediction of various design codes.

Normalized shear stress, V Shear stress ratios (V /V )
ns exp code
Beam ID
V V V V V V V
Exp BS8110 ACI 318 EC2 BS8110 ACI 318 EC2
P0 0.42 0.30 0.30 0.33 1.40 1.40 1.27
P200 0.61 0.53 0.41 0.55 1.15 1.49 1.11
P250 0.57 0.48 0.38 0.50 1.19 1.50 0.98
P300 0.52 0.45 0.37 0.47 1.16 1.41 1.11
N0 0.45 0.34 0.32 0.37 1.32 1.41 1.22
N200 0.55 0.53 0.40 0.55 1.04 1.38 1.00
N250 0.53 0.49 0.38 0.50 1.08 1.39 1.06
N300 0.49 0.40 0.37 0.47 1.23 1.32 1.04

Figure 6. Effect of spacing of shear reinforcement on normalized shear stress (Vns).

without shear reinforcement. The variation of the shear loads are also compared with the predictions of the
stresses in relation to the amount of shear reinforcement methods contained in the BS 8110, ACI 318-08 and the
was found to be non-proportional. That notwithstanding, EC 2. From the study, the following conclusions are
the shear stresses decreased with decreasing amount of drawn on the behaviour of PKS concrete beams in shear.
shear reinforcement. As seen from Figure 7, the The general behaviour of PKS concrete beams in shear
experimental shear stress results were higher than the is comparable to that of an equivalent NWC beam. The
code predictions, indicating that the codes are more addition of shear reinforcement showed a higher
conservative with the design of reinforced PKSC beams. enhancement of the shear capacity of the PKS beams
compared to corresponding NWC beams. The failure
modes of PKS concrete beams were brittle in nature as a
Conclusion result of diagonal tension. The post-diagonal shear
resistance of PKSC beams are higher than that of
The shear resistances of NWC and PKS concrete beams corresponding NWC beams, especially beams with shear
with and without shear reinforcement were described reinforcement. Nevertheless, the ultimate shear strength
based on the experimental results. The results of these of NWC beams were higher than that of corresponding
tests are discussed with particular attention to cracking PKS concrete beams. EC 2 gives the closest prediction
loads, maximum crack width, deflection, ultimate loads of the shear capacity of PKSC beams compared to the
and the effect of shear reinforcement on both PKS shear models of BS 8110 and ACI 318-08 which tends to
concrete and NWC beams. The experimental failure under-estimate the shear capacity of PKS concrete